Protective coating for cans



have been applied to Patented Nov. 3, 1931 C. RICHARDSON, F MENLO PARK,

CALIFORNIA, ASSIGNOR T0 CALIFORNIA PACKING CORPORATION, OF SAN FRANCISCO, CALIFORNIA, A CORPORATION OF N EW YORK PROTECTIVE COATING r03 cans No Drawing. Application filed J'une 19,

This invention relates to a protective coating for metallic containers and a process of coating metallic containers or parts of containers, and the invention particularly refers to a protective coating to be'used upon cans which are used as containers for food products, particularly food products of acidic nature.

The cans employed as containers for food products are generally formed of an iron or steel plate of metal having a comparatively thin coating of tin. The coating of tin is not entirely continuous over the iron. In packing of foods in containers of this kind, corrosion takes place upon both the exposed surface of the iron and to a lesser extent, upon the tin of the containers.

Heretofore, various lacquers and coatings cans used in canning foods. The lacquers employed for internal coatings have generally required a baking operation in order to fix the lacquer to the cans. For this reason. the lacquer must be applied to the metal before the same is made into a can, as the heat of the baking operations will fuse the solder used in making the can. In the operations of forming the lacquered plates into the can, there are created places where the can is imperfectly covered with the lacquer. In practical use, with lacquered cans of this kind, theexposed portions are subject'to corrosion and the food oackers have experienced difliculty in hold- ,ng food products in a can for long periods )f time, particularly when packing a food of LCldlC nature. It is also found that it is )ractically impossible to cover the can with coating so that there will he no exposed porions of the metal of the can, since the hanlling of the can in the packing operations nherently results in various portions of the irotective coating being broken.

The packing art has also employed what is enerally termed re-enameled cans, which re cans made from lacquered plates and then )ated with further lacquering solutions and ibjected to a compromised baking, which apartialextent, fixes the lacquer to the 1-11, but which baking is terminated before 1e solder of the can would become fused.

1?;28. Serial No. 286,716.

operation, use ,to afprotection to the can against In packing of foods, it is also desired to provide an exterior lacquer or coating for the cans after the food is packed. therein. The lacquers now in use for such purposes require that the exterior surface of the can be dry before the lacquer may he applied, with the result that the exterior lacquerin of the can interrupts the continuity of the packing operations. The necessity of using dried cans in lacquering the same which has been experienced with the methods of laequering or covering cans, heretofore used, has also in- Jured the economical re-lacquering of rusty cans. When it is found that cans have become rusted, under the present practice, the rust can not be removed from the cans by the use of solutions of rust solvents without the cans thereafter being immediately dried and lacquered. This is for the reason that drying of cans adds considerable expense to the operation, and rusty cans, therefore, can not be economically treated by aqueous solutions of rust removing agents when ordinary lacquers are to be used.

It is the general object of the present inventlon to provide a protective coating for metallic containers or cans and a method of applying such a coating which is adapted to provide a more effective protection for the metallic container or can from corrosion than the coatings or methods of protecting cans that have heretofore been employed.

Another object of the present invention is to prov de a coating and method of coating cans WhlCh may be applied to the cans or metallic containers while the cans or metallic containers are in the wet condition, to the end that .the lacquering operations may be used for lacquering the exterior of the cans without interrupting the continuity of the packing operations, and to the end that, rusty cans or other cans may be more readily cleaned by aqueous solutions of rust solvents, and lacquered without intermediate drying.

Another object of the present invention is to provide a protective coating for metallic containers which is adapted to withstand corcorrosion to a very marked extent.

rosion and includes an agent for inhibiting corrosion of the can which is capable of functionin to protect both the area of the can covere by the coating and any adjacent area where the coating may be imperfect.

As a basis of the present invention, it has been found that coatings including casein or casein products, when employed on metallic containers or cans are capable of acting to protect the metallic containers or cans from It is also found that the casein coatings may be applied to the cans when the cans are wet, and the coating may be solidified or fixed onto the can without the necessity of subjecting the same to a baking operation. By employing a coating including casein, it is, therefore, possible to coat the can entirely after the plate'has been manufactured into a can, rather than to apply the coating to the plate metal before it is made into a can.

It is also found possible to readily apply to the can any number of desired coatings. It is also found possible to coat the exterior of canned goods without interrupting the continuity o packing operations, as the coating may be applied and fixed in position upon a wet can. It is also found possible to more easily cover and coat cans which have become It is found that when casein is employ-ed in a coating for the inner surface of a can, that the casein is capable of inhibiting the corrosive action of acids upon the metal of the can and this property of casein, is effective not only to protect the portion of the surface of the can which'is actually covered by the casein coating, but the acids of the foods act upon the coating so as to extend the corrosive inhibiting action over a considerable adjoining area, so that casein containing coating operates to inhibit corrosion even where there are slight breaks in the original casein coating.

Since in the operation of canning foods, it is practically impossible to prevent a break in any protective coating that is applied to the cans, the ability of the protective coating of the present invention to exert an influence towards inhibiting corrosion of the unprotected portions of the cans, permits the coating of the present'invention to eliminate corrosicn to a substantially marked extent, over that which can be accomplished by coatings or lacquers heretofore employed.

Various further objects and advantages of the present invention will be understood from the description of a preferred form or example of a protective coating for metallic containers and process of producing the same, which embodies the present invention. There is,'therefore, hereinafter pointed out apreferred form or example of the invention.

In the preferred form of the invention,

I there is employed as a coating solution, a solution of casein. As an example'of a preferred solution, I have employedfapproximately 2% of casein which is rendered soluble in water by the addition of ammonia. Ammonia has the advantage over other solvents of casein being volatile, so that after the application of the coating solution to the can, any excess ammonia may be readily dried or evaporated from the coatin without leaving an undesirable deposit. 1? is apparent that other volatile solvents such as other volatile bases. The casein solu-' tion may also contain an agent employed in the solution for the purpose of facilitating drying of the coating of the solution. For this purpose, I employ in the preferred solution, alcohol and the alcohol may constitute, for example, 5 to 40% of the solution. The

of casein might be employed,

quantity of alcohol employed in this solution 1 will, of course, depend upon the drying time desired in a coating formed by this solution.

A gallon of the solution thus described, is capable of coating the inside andoutside of approximately 1000 ordinary No. 1 fruit cans. The operations of coating a can with the solution may be carried out in any desired or preferred manner. For example, the cans may be dipped in "the coating solution or sprayed with the solution. After the solutionhas been applied to the *can, if desired, the solution may be dried and the can used in such form for packing acidic goods. As a further alternative, the can, after being coated with the casein solution may be-dried or partially dried, and the coating then covered with the coating of a difierent lacquer such as the enamel lacquers now generally used for coating cans. The coating is, however, preferably fixed so that it is insoluble against acid and alkali solutions by subjecting the coating to the action of a condensing agent, such as formaldehyde.

The preferred method of condensing the casein coating to an insoluble form is by subjecting the coating to formaldehyde gas in a heated chamber, so that coating of the cans is dried simultaneously with the reaciion of the formaldehyde on the coating. In operation, for example, the cans are introduced with the coating solution thereon, in a chamber held at a temperature of F. which contains an atmosphere of formaldehyde and the cans are maintained in said chamber until the coating has become dried At the end of such operation, the can carries a casein covering which is insoluble in acid and alkali solutions and capable of protect ing the metal of the can from corrosion 0'. various kinds.

A coating secured by the process of th present invention is not only very efi'ective but a very slight coating can'be used wit] satisfactory results, which, being transparen does not alter the appearance of the cans. I

desired, however, there may be added to the coating various materials for coloring the can or giving the coating any other desired property. On a microscopic examination of a can coated in accordance with the present invention, it will be found that, as with other coatings, there may be some uncovered portions of the can. In use, however, the casein coating is found capable of inhibiting or retarding corrosion to a very marked degree, even at such uncovered portions of the can.

After subjecting the coating to the action with formaldehyde gas and drying the coating. the coating is insoluble towards more of the solution of casein, so that the operation of coating cans may be repeated, if desired, giving a further coating. After the desired thickness of coating on the inside of the can has been obtained, the can may be employed for packing food products in any usual or preferred manner. It is understood that the covers of the cans may be coated separately from the balance of the cans and the coating of the present invention may be applied directly over the gaskets of the covers Without injuring theeffectiveness of such gaskets. If desired, after sealing the food in the can additional coverings of the lacquer of the present invention may be applied Without previously drying the can. This permits the application of exterior lacquer without interrupting the continuity of the packing operations. The heat in the cans resulting from the packing operations, is suflicient to affect the drying of the coating which is ipplied to the exterior of'the cans.

As stated hereinabove.it is not essential hat the coating of casein or casein-containng composition be hardened by the use of .ormaldehyde or similar material. Also, the )roteetive coating of casein need not be com- )letely continuous but instead may form a liscontinuously associated coating, as even ucl discontinuous casein coating inhibits orrosion of a metallic container when acidic ood products are packed in such container.

The casein coatings of this invention are a be distinguished from continuous enamels r mechanical barriers which the prior art as attempted to use in that the casein coatigs are permeable whereas the enamels of ie prior art are designed to be continuous, npermeable and thus act as electric insu- LtOI'S between the acidic contents and the ietallic container. The enamels of the prior ."t have been of a brittle character and durig handling. cracks form in such enamel. orrosion is primarily due to potential set between the metallic container and the intents. Corrosion is therefore pronounced cracks in the enamel coatings because the it-ire potential difference between the coniner and the contents is concentrated at ,ch cracks. Pinholes are thus very rapidly rmed in the container.

The casein coatings of this invention are still slightly soluble after they have been dried. Furthermore, these coatings are flexible and permeable. When an acidic food product is placed in a metallic container whose interior surfaces are coated with the *asein coating of this invention, the casein coating is converted into a hydrated colloidal protective layer which is permeable to potential differences between the acidic contents and the metal of the container. Furthermore, it has been found that the casein coatings in the hydrated colloidal state which they assume in contact with acidic contents are capable of adsorbing hydrogen (which is usually liberated during corrosion) and thus form a counter electro-motive force increasing polarization of the metallic container and thus inhibiting further corrosion.

The casein coatings of this invention therefore, differ in mode of operation from the proposed enamels and insulating coatings of the prior art in that the casein coating does not attempt to prevent all corrosion, but instead permits a very small amount of corrosion to take place as soon as a can is filled and then inhibit further corrosion by reason of the polarizing character of the casein coating. Furthermore, the casein coatings ofthis invention increase the life of the cans very appreciably as leakage of potential is not localized to cracks of the coating but instead any such leakage is uniformly distrib uted over the entire area of the can.

In the process of applying the coating of 100 the present invention to cans, it is important that cans be as free as possible from greases or oils which will prevent wetting of the cans bythe casein solution.

As cans are ordinarily obtained from the manufacturer, they are covered with palm oil, which to an extent, interferes with the application of the protective coating. It has been found that the cans may be more perfectly cleaned from greases and oils by application to-the cans of a solution of trisodium phosphate or other mildly alkaline salts, by spraying the cans or immersing the cans in a solution of tri-sodium phosphate. The cans may then be rinsed, and it will be found that the casein solution adheres satisfactorily thereto.

An important advantage of the present invention resides in the fact that the lacquer solutions may be applied to the metal surfaces of the can while the can is warm and wet with water. The ordinary lacquers employed in canning operations require the can to be dry. .The operation of canning foods as now carried out nearly always is one long continuous process in which the open cans as packed, are first drained of superfluous water, then passed through a machine which adds liquid (sugar solutions, brine, oil, etc.), thence, through a steam box called an extomatically warm and haust box which removes substantially all the air,thence, to an automatic capping machine which applies the top and hermetically seals the cans, and thence, to an automatic cooker and cooler in which the cans are cooked in steam or water and then cooled in running water. From this cooler, the cans issue auwet. Normally, if the cans are to be lacquered on the outside, it is necessary to either, one, provide a long runway and air blast for drying and further cooling the cans, or two. to stack the cans on trays until dry and cool. Either method of procedureadds considerable cost to the canning operations.

The lacquering or coating process of the present invention, therefore, has the advantage that the lacquer may be applied to the warm and wet cans without interrupting the canning operations to stack the cans to dry and cool the same and without requiring the packing machinery to include a long runway and blowers for drying and cooling the cans.

If desired, when lacquering canned goods that have been previously packed, to cover the outside of the can, the goods may be handled in the following manner:

The canned goods are first heated to a temperature, of for example, F., and then dipped in the lacquer solution, before described, to which may be added the formaldehyde desired for the operation of rendering the film more insoluble, so that the operations of coating the can apply also the formaldehyde necessary for the condensing reaction and the initial heat of the can provides the necessary heat for the evaporation of the coating to dryness. When it is desired to add the formaldehyde to the original casein solution, it may be added if desired, in the form of para-formaldehyde, 01' formalin. The para-formaldehyde may be added to the solution in proportions, of, for example, 10% of the weight, of the casein in the solution.

While the particular formof the invention herein described is well adapted to carry out the objects of the invention, it is understood that various modifications and changes may be made without departing from the invention. and the invention is of the scope set forth in the appended claims. Where in the claims it is stated that the coating is of casein, it is intended to include a coating including both casein and compounds of casein resulting from the addition of alkalis used to render the casein soluble.

. I claim: 7

1. In a process of preventing and inhibiting corrosion of metallic containers'for acidic food products, tinuously associated permeable coating of casein on the interior surfaces of cans, casein coating being adapted to adsorb hythe step of forming a'discon-' said dro en and increase polarization at the can sur aces.

2. The process of forming. a protective coating on the metallic container which comprlses forming a discontinuously associated permeable coating of casein on the inner surface of said container, said casein coating being adapted to absorb hydrogen and increase polarization on the container surfaces, thereby functioning to inhibit'corrosion of the coated areas of the container without accelerating the corrosive attach upon the uncoated areas thereof.

3. In a process of preventing and inhibiting corrosion of metallic containers for acidic products, the step of forming a permeable coating of casein on the interior surfaces of the cans, said casein coating being adapted to permit polarization of the metallic surface by forming a colloidal gel when brought in contact with the acidic products.

4. As a new article of manufacture, a metallic receptacle provided with a protective coating of casein, adapted to form a hydrated colloidal protective layer between the metallic receptacle and contents of such receptacle, said hydrated colloidal protective layer being capable of absorbing hydrogen and increasing polarization of the metallic receptacle.

- 5. As an article of manufacture, a metallic container for acidic products comprising, a metallic container provided with a dis continuously associated permeable and flexible coating of casein on the interior surfaces of a container, said casein coating being adapted to adsorb hydrogen and increasr polarization at the container surfaces.

6. As an article of manufacture, a metallic container for acidic products comprising a metallic container provided with a discontinuously associated permeable and flexible coating of casein on the interior surfaces of a container, said casein coating being adapted to adsorb hydrogen and increas1 polarization at the container surfaces whereby corrosion of coated areas of sair. container is inhibited without accelerating corrosive attack' upon the uncoated area therof.

7. As an article of manufacture, a metal lic containerfor acidic products comprising. a metallic container provided with a discon tinuously associated permeable coating 0 casein on the interior surfaces of the con tainer, said casein coating being adapted t form a colloidal gel when brought in contac with acidic food products.

8. As an article of manufacture, a metal lic container for acidic products comprising a metallic container provided with a discor tinuously associated permeable coating c casein on the interior surfaces of the C01 tainer, said casein coating being adapted 1 form a colloidal gel when brought in C01 of the container is inhibited without accelerating corrosive attack upon the uncoated areas thereof.

Signed at San Francisco, California this 12th day of June 1928.

ALAN c. RICHARDSON. 

